Rocker arm structure

ABSTRACT

A rocker arm arranged between a camshaft and a valve which opens/closes an intake port or an exhaust port. A rolling bearing for supporting a support hole formed in a rocker arm is of a type of bearing wherein a plurality of needles are mounted on an inner side of an outer peripheral shell, and the outer peripheral shell is fixed to the support hole, the cam follower includes an outer ring which is supported on a periphery of a cam follower shaft which is held by a pair of left and right arm portions of the rocker arm by way of a plurality of rollers, a position of the outer ring and a position of the rollers in the lateral direction are restricted by the arm portion, and an outer diameter of the outer ring is set equal to or less than an inner diameter of the support hole.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2011-024854 filed on Feb. 8, 2011 the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the rocker arm structure.

2. Description of Background Art

For reducing friction in a valve system, efforts have been made to miniaturize a size of a cam follower portion or to reduce a weight of the cam follower portion by forming the cam follower into a shell structure having needle bearings. See, for example, JP-A-2004-346760. However, to realize the comprehensive miniaturization and reduction of weight of the cam follower portion or the reduction of friction in a valve system in the combination of the cam follower portion and a support portion of a rocker arm per se, further improvements with respect to the rocker arm structure are necessary.

SUMMARY AND OBJECTS OF THE INVENTION

The task of the present invention lies in the reduction of friction of a rocking part or a rotary part, the miniaturization of a rocker arm and the miniaturization of a decompression cam.

The present invention has been made to overcome the above-mentioned drawbacks. According to an embodiment of the present invention, a structure of a rocker arm (57) of a valve system is provided which is arranged between a camshaft (53) and a valve (45, 47) which opens/closes an intake port (42) or an exhaust port (43) in an internal combustion engine (15).

The rocker arm (57) includes a support hole (67) for enabling rockable supporting of the rocker arm 57 by way of a rocker shaft (56) which is supported on a cylinder head (22). A cam follower (58) receives a pushing force from a cam (54A, 54B) of the camshaft (53). The support hole (67) and the cam follower (58) are supported by a rolling bearing structure.

A rolling bearing (68) which supports the support hole (67) is of a type of bearing where a plurality of needles (70) are mounted on an inner side of an outer peripheral shell (69) with the outer peripheral shell (69) being fixed to the support hole (67).

The cam follower (58) includes an outer ring (74) which is supported on a periphery of a cam follower shaft (72) which is held by a pair of left and right arm portions (66) of the rocker arm (57) by way of a plurality of rollers (73). The positioning of the outer ring (74) and the positioning of the rollers (73) in the lateral direction are restricted by the arm portion (66).

An outer diameter (Dr) of the outer ring (74) is set equal to or less than an inner diameter (Dh) of the support hole (67).

According to an embodiment of the present invention, in the structure of the rocker arm (57), an escape portion (78) is formed on the rocker arm (57) so that the rocker arm (57) does not obstruct the movement of a distal end of a cam nose (54 a) when the camshaft (53) is rotated.

According to an embodiment of the present invention, an exhaust-side rocker arm (57B) out of the rocker arm (57), a decompression device (65) which is engaged with the exhaust-side rocker arm (57B) is mounted on the camshaft (53), a decompression shaft portion (86) of a decompression cam (84) which the decompression device (65) includes is supported on an inner side of an outer peripheral surface of a base circle portion (54 b) of the exhaust cam (54B) on a side opposite to the cam nose (54 a). The decompression cam (84) applies a decompression action to a projecting portion (66Ea) of an arm portion (66E) of the exhaust-side rocker arm (57B).

According to an embodiment of the present invention, the arm portions (66) of the exhaust-side rocker arm (57B) are formed such that, compared to a width of the arm portion (66E) on a side where the arm portions (66) are brought into contact with the decompression cam, a width of the arm portion (66I) on the other side is set small.

According to an embodiment of the present invention, the internal combustion engine (15) is an internal combustion engine (15) where the camshaft (53) and the rocker shaft (56) are arranged in front of a combustion chamber (40), and in a planar projection view as viewed in the cylinder axis direction (FIG. 5), with respect to each rocker arm (57), a size (F) between an axis (56 x) of the rocker shaft (56) and an axis (72 x) of the cam follower shaft (72) is set smaller than a size (C) between the axis (56 x) of the rocker shaft (56) and an axis (53 x) of the camshaft (53).

According to an embodiment of the present invention, the rocker shaft (56) is fixed to the cylinder head (22) by fastening in a non-rotatable manner.

According to an embodiment of the present invention, the cam follower shaft (72) of the cam follower (58) is formed of a solid small-diameter shaft, and a thickness of the outer ring (74) in the radial direction is set smaller than a diameter of the roller (73).

According to an embodiment of the present invention, the respective rocker arms (57) arranged adjacent to each other are formed such that a width of the arm portion (66) on a left side and a width of the arm portion (66) on a right side differ from each other, and the width of the inner arm portion (66I) is set smaller than a width of the outer arm portion (66E).

According to an embodiment of the present invention, the internal combustion engine (15) is a frontwardly-inclined engine where a cylinder axis extends frontwardly and upwardly, and oil supply guide walls (76A, 76B) which lead to an end surface of the support hole (67) of the rocker arm (57) from the rocker shaft support boss portion (55) in a vehicle mounted state are formed in the vicinity of the rocker shaft support boss portions (55) of the cylinder head (22).

According to an embodiment of the present invention, the oil supply guide walls (76A, 76B) are arranged close to the rocker shaft support boss portions (55) which are positioned laterally outside the intake-side rocker arm (57A) and the exhaust-side rocker arm (57B) which are arranged parallel to each other in the lateral direction toward an oil supply hole (75) formed in the cylinder head cover (23).

According to an embodiment of the present invention, both the support hole (67) and the cam follower (58) have the rolling bearing structure. Thus, a friction loss can be reduced. Further, a size of the outer diameter (Dr) of the outer ring (74) of the cam follower (58) is set equal to or less than a size of the inner diameter (Dh) of the support hole (67). Thus, a wall thickness (80) between the support hole 67 and an outer ring housing portion (79) can be ensured while keeping a length of the rocker arm (57) short. In addition, the required rigidity for press-fitting of the shell-needle-type rolling bearing (68) can be ensured. Further, circularity of an inner surface of the shell after press-fitting can be enhanced so that durability of the rocker arm (57) can be enhanced. Thus, the center of gravity of the rocker arm (57) can be made to approach a rocking fulcrum so that a load applied to the shell (69) can be reduced.

According to an embodiment of the present invention, the escape portion (78) is formed on the rocker arm (57) and hence, also in the cam follower (58) having the small-diameter outer ring (74). Thus, it is possible to prevent the cam nose (54 a) and the rocker arm (57) from being brought into contact with each other.

According to an embodiment of the present invention, a stepped portion between the projecting portion (66Ea) of the arm portion (66E) of the rocker arm (57) and the outer ring (74) can be made small. Thus, a projection amount of the decompression cam (84) from the outer peripheral surface of the base circle portion (54 b) of the exhaust cam (54B) can be made small whereby the decompression cam (84) can be miniaturized.

According to an embodiment of the present invention, compared to the width of the arm portion (66E) of the exhaust-side rocker arm (57B) on a side where the arm portions (66) are brought into contact with the decompression cam (84), the width of the arm portion (66I) of the exhaust-side rocker arm (57B) on the other side is set small. Thus, the arm portion to which a load is not applied can be made light-weighted.

According to an embodiment of the present invention, the cam follower shafts (72) of the respective rocker arms (57) are arranged close to the corresponding rocker shafts (56). Thus, a length of the rocker arm (57) can be shortened. Further, a lever ratio (a ratio between the distance from the rocker shaft (56) to the tappet screw (59) and the distance from the rocker shaft (56) to the cam follower shaft (72)) can be set large. Thus, a cam height can be set low. Still further, since irregularities in the direction of a cam-crest reaction force which is applied to the rolling bearing (68) by way of the rocker arm (57) can be set in the fixed direction, the irregular movement of the rocker arm (57) can be suppressed. Accordingly, vibrations of the respective parts generated by rocking of the rocker arm can be also reduced thus further enhancing the durability of the bearing.

According to an embodiment of the present invention, the rocker shaft (56) is fixed and hence, the needles inside the shell (69) can be surely rotated. Accordingly, it is possible to prevent the needles (70) and the rocker shaft (56) from local abutting caused by the co-rotation of the needles (70) and the rocker shaft (56). Thus, the durability of the needles (70) and the rocker shaft (56) can be enhanced.

According to an embodiment of the present invention, the size of the outer diameter (Dr) of the outer ring (74) can be set small. Thus, the rocker arm (57) can be miniaturized.

According to an embodiment of the present invention, a distance between the cam followers (58) of both rocker arms (57) can be decreased. Thus, a distance between the cams (54A, 54B) of the camshaft (53) can be decreased whereby the valve gear (51) can be miniaturized.

According to an embodiment of the present invention, due to the formation of the oil guide walls (76A, 76B), an amount of oil supplied to the rolling bearing (68) of the support hole (67) can be increased. Thus, the durability of the rolling bearing (68) of the support hole (67) can be enhanced.

According to an embodiment of the present invention, although the rocker shaft support boss portions (55) are separated from each other in the lateral direction toward the oil supply hole (75) of the valve system which is formed in the cylinder head cover (23) at one place positioned approximately right above the top portion of the intake valve (45), a lubrication oil which is supplied from the oil supply hole (75) and is splashed by hitting the tappet screw (59) on the top portion of the intake valve (45) is received by the supply oil guide walls (76A, 76B) and is supplied to the support hole (67) formed in the rocker arm (57). Thus, the lubrication oil can be supplied to the support hole (67) formed in the rocker arm (57) whereby an amount of the lubrication oil supplied to the rolling bearing (68) in the support hole (67) can be increased while simplifying the oil supply structure in the inside of the cylinder head cover (23).

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a left side view of a motorcycle according to one embodiment of the present invention;

FIG. 2 is a longitudinal cross-sectional left side view of a power unit;

FIG. 3 is a longitudinal cross-sectional view of a cylinder head and the constitution in the vicinity of the cylinder head as viewed from a left side;

FIG. 4 is a horizontal cross-sectional view of the cylinder head and the constitution in the vicinity of the cylinder head;

FIG. 5 is a view showing the inside of the cylinder head as viewed from a front side;

FIGS. 6( a) to 6(e) are views of an exhaust rocker arm;

FIG. 7 is an enlarged view showing the relative positional relationship between the exhaust rocker arm and a decompression device in a state where the decompression can be performed;

FIG. 8 is a perspective view of a decompression cam;

FIG. 9 is an enlarged view showing the relative positional relationship between the exhaust rocker arm and the decompression device in a state where the decompression is released;

FIG. 10 is a view showing the inside of the cylinder head as viewed from a front side in a state where the internal combustion engine is mounted on a vehicle; and

FIG. 11 is a cross-sectional view showing the rocker arm support structure according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a left side view of a motorcycle 1 according to one embodiment of the present invention. A vehicle body frame of the motorcycle 1 includes a head pipe, a main frame which extends rearwardly and downwardly from the head pipe, a pair of left and right rear frames which have one ends thereof connected to a rear portion of the main frame and extends rearwardly and upwardly, and a plurality of other frames. A front wheel 3 is pivotally supported on a lower end of a front fork 2 which is rotatably supported on the head pipe with a steering handle 4 being connected to an upper portion of the front fork 2. The motorcycle 1 includes a power unit 5. A rear wheel 7 is mounted on a rear axle 6 (FIG. 2) which projects to the right from a rear portion of the power unit 5 with the rear wheel 7 being driven by the power unit 5. An air cleaner 8 is arranged above the power unit 5. A synthetic-resin-made vehicle body cover 9 which includes a plurality of parts is mounted on the vehicle body frame with the vehicle body cover 9 covering the power unit and other equipments. A tandem seat 10 is mounted on an upper portion of the vehicle body.

FIG. 2 is a longitudinal cross-sectional left side view of the power unit 5. The power unit 5 of the motorcycle 1 is suspended from a power unit suspension bracket (not shown in the drawing) which is mounted on the rear frames by way of a hanger 11 which is integrally formed on a front portion of the power unit 5 and a support shaft 12. A rear shock absorber 14 (FIG. 1) is arranged between a bracket 13 which is mounted on a rear end portion of the power unit 5 and a bracket mounted on rear portions of the rear frames (not shown in the drawing). The power unit 5 is swingably suspended by these parts in a state where a cylinder axis of the power unit 5 is set in the slightly frontward and upward direction.

The power unit 5 includes an internal combustion engine 15 which is arranged on a front side of the power unit 5 and a power transmission device 16 which extends rearwardly from a left side of the internal combustion engine 15. The power transmission device 16 includes a V-belt-type continuously variable transmission 17 and a final reduction gear 18.

A shell body of the above-mentioned internal combustion engine 15 includes a crankcase 20, and a cylinder block 21, a cylinder head 22 and a cylinder head cover 23 which are joined to a front portion of the crankcase 20 in order in a frontward direction. The internal combustion engine 15 is a rocker-arm-type overhead-valve-type 4-stroke-cycle single cylinder water-cooled-type internal combustion engine. A throttle body 26 is mounted on an intake pipe 25 which is mounted on an intake port formed on an upper side of the cylinder head 22. Further, an air cleaner 8 (FIG. 1) is connected to a rear side of the throttle body 26. A fuel injection valve 27 is mounted on the above-mentioned intake pipe 25.

A drive shaft of the V-belt-type continuously variable transmission 17 is a crankshaft 28 of the internal combustion engine 15 itself, and a drive pulley 29 of the V-belt-type continuously variable transmission 17 is mounted on a leftwardly extending portion of the crankshaft 28. A driven shaft 30 of the V-belt-type continuously variable transmission 17 is pivotally and rotatably supported on a transmission case 33 by way of a bearing. A driven pulley 31 is mounted on the driven shaft 30 by way of a centrifugal clutch. An endless V belt 32 extends between the drive pulley 29 and the driven pulley 31.

The final reduction gear 18 is arranged in a rear portion of the V-belt-type continuously variable transmission 17. An input shaft of the final reduction gear 18 is the above-mentioned driven shaft 30. A rear axle 6 to which the rear wheel 7 (FIG. 1) is integrally joined is rotatably and pivotally supported on the transmission case 33 and a gear case 34. A torque of the driven shaft 30 is transmitted to the rear axle 6 by way of a plurality of gears. A rotational speed of the rear axle 6 is largely reduced compared to a rotational speed of the driven shaft 30 so that the rear wheel 7 which is joined to the rear axle 6 is driven at a reduced rotational speed.

FIG. 3 is a longitudinal cross-sectional view of the cylinder head 22 of the internal combustion engine 15 and the construction in the vicinity of the cylinder head 22 as viewed from a left side, FIG. 4 is a horizontal cross-sectional view of the above-mentioned cylinder head 22 and the construction in the vicinity of the cylinder head 22, and FIG. 5 is a view showing the construction in the inside of the cylinder head 22 by removing the cylinder head cover 23 as viewed from a front side. In explaining this embodiment in conjunction with these drawings, the frontward direction is indicated by an arrow Fr, the upward direction is indicated by an arrow Up, the downward direction is indicated by an arrow Dn, the leftward direction is indicated by an arrow L, and the rightward direction is indicated by an arrow R in the drawings, and the explanation is made in accordance with these respective directions.

In these drawings, the cylinder head 22 is joined to the cylinder block 21 and the crankcase 20 using bolts 36A and nuts 36B, and the cylinder head cover 23 is joined to the cylinder head 22 using bolts not shown in the drawing. A piston 37 is slidably fitted in a cylinder bore 38 which is formed in a cylinder liner 24 inside the cylinder block 21. The above-mentioned piston 37 is connected to a crank pin (not shown in the drawing) of the crankshaft 28 (FIG. 2) by way of a connecting rod 39. When the piston 37 reciprocates, the crankshaft 28 is rotatably driven. A combustion chamber 40 is formed in a bottom surface of the cylinder head 22 in a state where the combustion chamber 40 faces a top surface of the piston 37 in an opposed manner. An ignition plug 41 is mounted on the cylinder head 22 in a leftwardly inclined manner with respect to an axis of the cylinder bore 38.

In FIG. 3, a curved intake port 42 which has an upstream end thereof opened upwardly and has a downstream end thereof opened into the combustion chamber 40 that is formed in an upper portion of the cylinder head 22. A curved exhaust port 43 has an upstream end thereof opened into the combustion chamber 40 and has a downstream end thereof opened downwardly that is formed in a lower portion of the cylinder head 22. In the cylinder head 22, an intake valve 45 which opens/closes an intake opening 44 of the combustion chamber 40 and an exhaust valve 47 which opens/closes an exhaust opening 46 of the combustion chamber 40 are slidably fitted in valve guides 48 respectively. The intake pipe 25 is connected to an upstream-end opening of the intake port 42, and the throttle body 26 (FIG. 2) is connected to an upstream end of the intake pipe 25. The fuel injection valve 27 (FIG. 2) is mounted on the intake pipe 25, and a distal end of the fuel injection valve 27 faces the intake port 42. An exhaust pipe (not shown in the drawing) is connected to a downstream end of the exhaust port 43.

In FIG. 3 and FIG. 5, the intake valve 45 and the exhaust valve 47 which are biased in the valve closing direction by valve springs 49 are driven in an open/closed manner by a valve gear 51 arranged in the inside of a valve gear chamber 50 which is formed of the cylinder head 22 and the cylinder head cover 23. In FIG. 3 and FIG. 4, in the inside of the valve gear chamber 50, one camshaft 53 is horizontally and pivotally and rotatably supported on the cylinder head 22 by way of a pair of ball bearings 52, and an intake cam 54A and an exhaust cam 53B are integrally formed on the camshaft 53. In FIG. 3, an intake-side rocker shaft 56A is mounted on the cylinder head 22 in an extending manner in front of and above the camshaft 53, and an exhaust-side rocker shaft 56B is mounted on the cylinder head 22 in an extending manner in front of and below the camshaft 53. In FIG. 3 and FIG. 5, an intake-side rocker arm 57A and an exhaust-side rocker arm 57B are pivotally supported on the intake-side rocker shaft 56A and the exhaust-side rocker shaft 56B in a rocking manner respectively. On one ends of the above-mentioned rocker arms 57A, 57B, cam followers 58 which are brought into contact with the above-mentioned cams 54A, 54B are pivotally supported respectively. A tappet screw 59 is mounted on other ends of the above-mentioned rocker arms 57A, 57B, and these tappet screws 59 are positioned by locking nuts 60 respectively. These tappet screws 59 are respectively brought into contact with top portions of the intake valve 45 and the exhaust valve 47 thus opening/closing the intake valve 45 and the exhaust valve 47 corresponding to the rotation of the camshaft 53.

In FIG. 4 and FIG. 5, a cam chain 62 extends between a driven sprocket wheel 61 which is mounted on a right end of the camshaft 53 which is directed in the horizontal direction and a drive sprocket wheel (not shown in the drawing) which is mounted on the crankshaft 28 in a state where the cam chain 62 passes the inside of a cam chain chamber 63. The cam chain chamber 63 is formed in a communicating manner with the crankcase 20, the cylinder block 21 and the cylinder head 22. A water pump 64 which is driven by the camshaft 53 is arranged adjacent to the cam chain chamber 63 formed in the cylinder head 22 shown in FIG. 4. In FIG. 3 and FIG. 4, a decompression device 65 is mounted on a left surface of the exhaust cam 54B.

FIGS. 6( a) to 6(e) are views showing the above-mentioned exhaust-side rocker arm 57B.

FIG. 6( a) is a view (right side view) of the exhaust-side rocker arm 57B as viewed in the direction indicated by an arrow A in FIG. 6( b);

FIG. 6( b) is a view of the exhaust-side rocker arm 57B as viewed from a front side (see FIG. 5);

FIG. 6( c) is a view (left side view) of the exhaust-side rocker arm 57B as viewed in the direction indicated by an arrow C in FIG. 6( b);

omitted from the drawing. The intake-side rocker arm 57A and the exhaust-side rocker arm 57B are arranged parallel to each other (FIG. 4 and FIG. 5). Each rocker arm 57 includes two arm portions 66 respectively. In the explanation made hereinafter, the outer arm portion is indicated by symbol 66E and the inner arm portion is indicated by symbol 66I in a state where the intake-side rocker arm 57A and the exhaust-side rocker arm 57B are arranged parallel to each other as described above. A point which makes the exhaust-side rocker arm 57B differ from the intake-side rocker arm 57A lies in that out of the pair of left and right arm portions 66E, 66I of the exhaust-side rocker arm 57B, a projecting portion 66Ea (FIG. 6( c)) which is subject to a decompression action of the decompression device 65 is formed on the outer arm portion 66E.

The intake-side rocker arm 57A and the exhaust-side rocker arm 57B have the same shape with respect to portions other than the projecting portion 66Ea which is subject to a decompression action of the decompression device 65. When it is unnecessary to explain both rocker arms 57A, 57B in a differentiated manner, both rocker arms 57A, 57B are simply referred to as “rocker arm 57”. Further, also with respect to the rocker shafts 56A, 56B and the cams 54A, 54B, when it is unnecessary to differentiate an intake side and an exhaust side from each other, these rocker shafts 56A, 56B and these cams 54A, 54B are simply referred to as “rocker shaft 56” and “cam 54” respectively.

The rocker arm 57 includes a support hole 67 for enabling the rockable supporting of the rocker arm 57 by way of the rocker shaft 56 and the cam follower 58 which receives a pushing force from the cam 54. The support hole 67 portion and the cam follower 58 portion are supported by the rolling bearing structure. A rolling bearing 68 which supports the support hole 67 is, as shown in FIG. 6( d), a so-called “shell-needle-type bearing” where a plurality of needles 70 are mounted on an inner side of an outer peripheral shell 69, and the outer peripheral shell 69 is fixed to the support hole 67 by press fitting. On the other hand, as shown in FIG. 6( e), the cam follower 58 includes an outer ring 74 which is supported on the periphery of a cam follower shaft 72 which is held by the pair of left and right arm portions 66E, 66I of the rocker arm 57 by way of plural rollers 73, and the position of the outer ring 74 and the position of the rollers 73 in the lateral direction are restricted by the arm portions 66E, 66I.

An outer diameter Dr of the outer ring 74 is set equal to or less than an inner diameter Dh of the support hole 67. More specifically, the outer diameter Dr of the outer ring 74 of the cam follower 58 is set small compared to a conventional outer ring. For example, when the inner diameter Dh of the support hole 67 is set to 13.4 mm, the outer diameter Dr of the outer ring 74 is set to 13.0 mm. Due to such outer diameter setting, in FIGS. 6( b) and 6(c), a wall thickness 80 between the support hole 67 and an outer ring housing portion 79 can be ensured while keeping a length of the rocker arm 57 short. Thus, required rigidity for press-fitting of the shell-needle-type rolling bearing 68 can be ensured and, further, circularity of an inner surface of the shell after press-fitting can be enhanced. Thus, the durability of the rocker arm 57 can be enhanced, and the center of gravity of the rocker arm 57 can be made to approach a rocking fulcrum so that a load applied to the shell 69 of the rolling bearing 68 in the support hole 67 which forms the rocking fulcrum can be reduced. Further, both the support hole 67 portion and the cam follower 58 portion have the rolling bearing structure. Thus, it is possible to reduce friction loss.

In FIG. 5, the rocker shaft 56 is fastened and fixed to the cylinder head 22 by means of a bolt 77 in a non-rotatable manner. Since the rocker shaft 56 is fixed to the cylinder head 22, the needles 70 in the inside of the shell 69 of the rolling bearing 68 which supports the rocker arm 57 can be surely rotated. Accordingly, it is possible to prevent the needles 70 and the rocker shaft 56 from local abutting caused by the co-rotation of the needles 70 and the rocker shaft 56 thus enhancing the durability of the needles 70 and the rocker shaft 56.

In a planar projection view as viewed in the cylinder axis direction shown in FIG. 5, with respect to the respective rocker arms 57, a size F between an axis 56 x of the rocker shaft 56 and an axis 72 x of the cam follower shaft 72 is set smaller than a size C between the axis 56 x of the rocker shaft 56 and an axis 53 x of the camshaft 53. In either rocker arm 57, the cam follower shaft 72 approaches the corresponding rocker shaft 56. Thus, a length of the rocker arm 57 can be shortened. Further, a lever ratio (a ratio between a distance from the rocker shaft (56) to the tappet screw (59) and a distance from the rocker shaft (56) to the cam follower shaft (72)) can be set large. Thus, it is possible to set a cam height low. Still further, since irregularities in the direction of a cam-crest reaction force which is applied to the rolling bearing (68) by way of the rocker arm (57) can be set in the fixed direction, the irregular movement of the rocker arm (57) can be suppressed. Accordingly, vibrations of the respective parts generated by rocking of the rocker arm can also be reduced thus further enhancing the durability of the bearing.

In FIG. 6( a) and FIG. 6( c), an escape portion 78 is formed on the rocker arm 57 such that the rocker arm 57 does not obstruct the movement of a distal end of a cam nose 54 a (FIG. 7) when the camshaft 53 is rotated. Due to the formation of the escape portion 78, also with respect to the cam follower 58 having the small-diameter outer ring 74, it is possible to prevent the cam nose 54 a and the rocker arm 57 from being brought into contact with each other.

FIG. 7 is an enlarged view showing the exhaust-side rocker arm 57B and the decompression device 65 which is mounted on a left surface of the exhaust cam 54B. In FIG. 4 and FIG. 7, the decompression device 65 includes a support shaft 81 which is fitted into and is mounted on an inner side of the cam nose 54 a of the exhaust cam 54B, a decompression weight 82 which is supported on the support shaft 81 in a rotatable manner about the support shaft 81, a decompression cam 84 which is supported in a rockable manner by way of a decompression shaft portion 86 which is inserted into an inner side of a base circle portion 54 b of the exhaust cam 54B on a side opposite to the cam nose 54 a, an operation pin 87 which is fixed to a distal end portion 82 a of the decompression weight 82 and is engaged with a groove portion 85 a formed on the decompression cam 84, and rocks the decompression cam 84, and a biasing means 88 which is mounted on the camshaft 53 so as to push a tail end portion 82 b of the decompression weight 82. The biasing means 82 includes a pushing member 89 which is fitted in and mounted on the camshaft 53 in a projecting and retracting manner, and a coil spring 90 which pushes the pushing member 89. The decompression weight 82 is supported in a cantilever manner on the above-mentioned support shaft 81 which is mounted in the cam nose 54 a in a projecting manner, and is rockable along a left surface of the exhaust cam 54B. The above-mentioned biasing means 88 also functions as a stopper corresponding to the maximum displacement of the decompression weight 82.

FIG. 8 is a perspective view of the decompression cam 84. The decompression cam 84 includes a large-diameter decompression body portion 85 and a small-diameter decompression shaft portion 86 which projects from a right end of the decompression body portion 85 coaxially with an axis of the decompression body portion 85. The decompression shaft portion 86 is rotatably fitted into and mounted in a circular hole which is formed in an inner side of the base circle portion 54 b (on a side opposite to the cam nose 54 a) of the exhaust cam 54B, and the body portion 85 projects from a left surface of the exhaust cam 54B (FIG. 4). A groove portion 85 a which extends in the radial direction from the center to an outer periphery is formed on a left end portion of the body portion 85. A right end portion of the body portion 85 is formed into a cutaway portion 85 b by cutting away an arcuate portion along a chord portion of the arcuate portion. The above-mentioned groove portion 85 a and the cutaway portion 85 b have the predetermined positional relationship. A body cylindrical surface 85 c having no cutaway portion is a portion which performs a decompression action.

FIG. 7 shows the decompression device 65 in a state where the decompression device 65 can perform a decompression action. When the internal combustion engine is stopped or when the internal combustion engine is rotated at a low speed immediately after the internal combustion engine is started, the camshaft 53 is also stopped or is rotated at a low speed, and a centrifugal force which acts on the decompression weight 82 is zero or small. Accordingly, the pushing member 89 pushes the tail end portion 82 b of the decompression weight 82. Thus, the decompression weight 82 is pushed in the direction that the decompression weight 82 approaches the camshaft 53. Here, a side wall of the groove portion 85 a of the decompression cam 84 is pushed by the operation pin 87 of the decompression weight 82 which is engaged with the decompression cam 84 thus bringing about a state where the body cylindrical surface 85 c having no cutaway portion projects outwardly from an outer peripheral surface of the base circle portion 54 b of the exhaust cam 54B. When the camshaft 53 is rotated at a low speed, the body cylindrical surface 85 c having no cutaway portion pushes a projecting portion 66Ea of the outer arm portion 66E of the exhaust-side rocker arm 57B and opens the exhaust valve 47 by way of the tappet screw 59 in a compression stroke at an initial stage of starting the internal combustion engine. Such a state is referred to as a decompression state.

FIG. 9 shows the decompression device 65 in a decompression action release state. When an engine rotational speed is increased after the internal combustion engine 15 is started, a rotational speed of the camshaft 53 also exceeds a predetermined rotational speed. Accordingly, the decompression weight 82 is rotated in the direction that the decompression weight 82 is moved away from the camshaft 53 against a pushing force of the biasing means 88, and the operation pin 87 pulls a side wall of the groove portion 85 a of the decompression cam 84 thus rotating the cutaway portion 85 b to a position where the cutaway portion 85 b is directed in the outer peripheral direction of the base circle portion 54 b of the exhaust cam 54B. As a result, the body cylindrical surface 85 c having no cutaway portion of the decompression cam 84 is brought into a state where the body cylindrical surface 85 c does not project from the outer peripheral surface of the base circle portion 54 b of the exhaust cam 54B. Accordingly, the decompression cam 84 cannot push up the projecting portion 66Ea of the outer arm portion 66E of the exhaust-side rocker arm 57B. Thus, the exhaust valve 47 is brought into a valve closed state in a compression stroke. Such a state is referred to as a decompression action release state.

In FIG. 7, the above-mentioned decompression cam 84 applies a decompression action to the projecting portion 66Ea of the outer arm portion 66E of the exhaust-side rocker arm 57B. A stepped portion between the projecting portion 66Ea and the outer ring 74 of the cam follower 58 is set small. Accordingly, a projection amount of the decompression cam 84 from the outer peripheral surface of the base circle portion 54 b of the exhaust cam 54B can be set small whereby the decompression cam 84 can be miniaturized.

In FIG. 6( b), out of the pair of arms 66 of the exhaust-side rocker arm 57B, a width of inner arm portion 66I is set smaller than a width of the outer arm portion 66E which is brought into contact with the decompression cam 84. Accordingly, the arm portion to which a load is not applied can be made light in weight.

In FIG. 6( e), the cam follower shaft 72 of the cam follower 58 is formed of a solid small-diameter shaft, and a thickness t of the outer ring 74 in the radial direction is set smaller than a diameter d of the roller 73. Accordingly, the size of the outer diameter Dr of the outer ring 74 (FIG. 6( a)) can be set small. Thus, the rocker arm 57 can be miniaturized.

In FIGS. 4 and 5, with respect to the respective neighboring rocker arms 57, a width of the arm portion 66 on a left side is set different from a width of the arm portion 66 on a right side. More specifically, the inner arm portion 66I is formed narrow in width as compared to the outer arm portion 66E. Accordingly, a distance between the cam followers 58 of both rocker arms 57 can be decreased. Thus, a distance between the cams 54A, 54B mounted on the camshaft 53 (FIG. 4) can be decreased whereby the valve gear 51 can be miniaturized.

FIG. 10 is a view showing the inside of the cylinder head 22 as viewed from a front side by removing the cylinder head cover 23 in a state where the internal combustion engine 15 is mounted on the vehicle. Although FIG. 10 is the same view as FIG. 5, FIG. 5 is drawn by making the lateral direction therein agree with the lateral direction in FIG. 4 so that the vertical direction in FIG. 5 is opposite to the actual vertical direction. FIG. 10 is drawn such that the vertical direction in FIG. 10 agrees with the actual vertical direction. Accordingly, the lateral direction in FIG. 10 is opposite to the lateral direction in FIG. 4 and FIG. 5.

In FIG. 10, to lubricate the devices in the inside of the valve gear chamber 50, a lubrication oil is injected toward the top portion of the intake valve 45 from the oil supply hole 75 above the valve gear chamber 50 (FIG. 3). The lubrication oil is hit by the tappet screw 59 so that the lubrication oil is splashed in all directions whereby the lubrication oil is splashed to surfaces of the surrounding devices and walls and flows down along the surfaces. Supply oil guide walls 76A, 76B which receive the lubrication oil flowing down from above and guide the lubrication oil to the support holes 67 formed in the rocker arms 57A, 57B from rocker shaft support boss portions 55 are arranged close to the rocker shaft support boss portions 55 of the cylinder head 22.

Due to the formation of the supply oil guide walls 76A, 76B, an amount of oil supplied to the rolling bearing 68 in the support hole 67 can be increased and hence, the durability of the rolling bearing 68 can be enhanced. A bold-line arrow in the drawing shows the flow of lubrication oil which reaches the end surface of the support hole 67 from the oil supply hole 75 above the valve rear chamber 50.

In FIG. 10, the supply oil guide walls 76A, 76B extend toward the oil supply hole 75 formed in the cylinder head cover 23 (FIG. 3), and are arranged close to the rocker shaft support boss portions 55 which are arranged laterally outside the intake-side rocker arm 57A and the exhaust-side rocker arm 57B arranged parallel to each other. The oil supply hole 75 of the valve system is formed in the cylinder head cover 23 at one place positioned approximately right above the top portion of the intake valve 45 in a state where the internal combustion engine 15 is mounted on the vehicle. Although the rocker shaft support boss portions 55 are separated from each other in the lateral direction toward the oil supply hole 75 of the valve system, a lubrication oil which is supplied from the oil supply hole 75 and is splashed by hitting the tappet screw 59 on the top portion of the intake valve 45 is received by the supply oil guide walls 76A, 76B and is supplied to the support hole 67 formed in the rocker arm 57. Thus, the lubrication oil can be supplied to the support hole 67 formed in the rocker arm 57 whereby an amount of the lubrication oil supplied to the rolling bearing 68 in the support hole 67 can be increased while simplifying the oil supply structure in the inside of the cylinder head cover 23.

FIG. 11 is a cross-sectional view of a rocker arm support portion according to a second embodiment of the present invention. In this embodiment, the structure of the rocker arm is equal to the structure of the rocker arm 57 according to the first embodiment. A point which makes the second embodiment differ from the first embodiment lies in the structure of a rocker shaft 92 and the structure of a bearing which supports a support hole 67 of the rocker arm 57. The rocker shaft 92 is formed of a tubular material. In the same manner as the first embodiment, the rocker shaft 92 is fixed to a cylinder head using a bolt 77. An outer diameter of the rocker shaft 92 is set slightly smaller than an inner diameter of the above-mentioned support hole 67 formed in the rocker arm 57. In addition, a pair of O-rings 93 having a triangular cross section is mounted between the rocker shaft 92 and the support hole 67 instead of a roller bearing. Due to such a construction, the rocker arm 57 can be supported by the rocker shaft 92 in a floating state.

The rocker shaft 92 is formed of the tubular material has a center hole 92 a. Thus, both ends of the rocker shaft 92 open toward a valve gear chamber 50. An oil hole 92 b is in communication with a space defined by the above-mentioned pair of O-rings 93 and includes a triangular cross section from a center portion of the center hole 92 a. When an internal combustion engine 15 is operated, a lubrication oil splashed in a valve gear chamber 50 is supplied to a space defined by the support hole 67, the rocker shaft 92 and the pair of O-rings 93 having a triangular cross section through the center hole 92 a and the oil hole 92 b and hence, the rocker arm 57 is supported on the rocker shaft 92 in a floating manner whereby the rocker arm 57 can be smoothly rocked with respect to the rocker shaft 92. Due to the above-mentioned structure, a strike sound of a tappet screw 59 which is transmitted to a cylinder head 22 or the like from the rocker arm 57 by way of the rocker shaft 92 can be reduced.

As described in detail heretofore, the following advantageous effects are brought about by the above-mentioned embodiment.

Both the support hole 67 and the cam follower 58 have the rolling bearing structure and hence, a friction loss can be reduced. Further, a size of the outer diameter Dr of the outer ring 74 of the cam follower 58 portion is set small, that is, is set equal to or less than a size of the inner diameter Dh of the support hole 67. Thus, a length of the rocker arm 57 can be shortened, a wall thickness 80 between the support hole 67 and an outer ring housing portion 79 can be ensured, and the center of gravity of the rocker arm 57 can be made to approach a rocking fulcrum so that a load applied to the shell 69 can be reduced.

The escape portion 78 is formed on the rocker arm 57 so that the rocker arm 57 does not obstruct the movement of the distal end of the cam nose 54 a when the camshaft 53 is rotated and hence, also in the cam follower 58 having the small-diameter outer ring 74, it is possible to prevent the cam nose 54 a and the rocker arm 57 from being brought into contact with each other.

The decomposition device 65 is mounted on the camshaft 53, and the decomposition cam 84 is configured to apply a decomposition action to the projecting portion 66Ea of the arm portion 66E of the exhaust-side rocker arm 57B. A stepped portion between the projecting portion 66Ea and the outer ring 74 of the cam follower 58 can be made small. Thus, a projection amount of the decompression cam 84 from the outer peripheral surface of the base circle portion 54 b of the exhaust cam 54B can be made small whereby the decompression cam 84 can be miniaturized.

Compared to the width of the arm portion 66E of the exhaust-side rocker arm 57B on a side where the arm portion 66E is brought into contact with the decompression cam 84, the width of the arm portion 66I of the exhaust-side rocker arm 57B on the other side is set small. Thus, the arm portion to which a load is not applied can be made light in weight.

In a planar projection view as viewed in the cylinder axis direction, with respect to each rocker arm 57, a size F between an axis 56 x of the rocker shaft 56 and an axis 72 x of the cam follower shaft 72 is set smaller than a size C between the axis 56 x of the rocker shaft 56 and an axis 53 x of the camshaft 53. Accordingly, the cam follower shafts 72 can be set closer to the corresponding rocker shafts 56. Thus, a length of the rocker arm 57 can be shortened.

The rocker shaft 56 is fixed to the cylinder head 22. Thus, the needles 70 inside the shell 69 can be surely rotated. Accordingly, it is possible to prevent local abutting of the needles 70 and the rocker shaft 56 caused by the co-rotation of the needles 70 and the rocker shaft 56 thus enhancing the durability of the needles 70 and the rocker shaft 56.

The cam follower shaft 72 of the cam follower 58 is formed of a solid small-diameter shaft, and a thickness of the outer ring 74 in the radial direction is set smaller than a diameter of the roller 73. Thus, the rocker arm 57 can be miniaturized.

The respective rocker arms 57 arranged adjacent to each other are formed such that the width of the inner arm portion 66I is set smaller than the width of the outer arm portion 66E. Thus, a distance between the cam followers 58 can be decreased whereby the valve gear 51 can be miniaturized.

The oil supply guide walls 76A, 76B which lead to an end surface of the support hole 67 of the rocker arm 57 from the rocker shaft support boss portion 55 are formed. Thus, an amount of oil supplied to the rolling bearing 68 of the support hole 67 can be increased whereby the durability of the rolling bearing 68 of the support hole 67 can be enhanced.

Although the rocker shaft support boss portions 55 are separated from each other in the lateral direction toward the oil supply hole 75 of the valve system which is formed in the cylinder head cover 23 at one place positioned approximately right above the top portion of the intake valve 45, a lubrication oil which is splashed by hitting the tappet screw 59 on the top portion of the intake valve 45 is received by the supply oil guide walls 76A, 76B and is supplied to the support hole 67 formed in the rocker arm 57 and hence, an amount of the lubrication oil supplied to the rolling bearing 68 in the support hole 67 can be increased while simplifying the oil supply structure in the inside of the cylinder head cover 23.

The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. A structure of a rocker arm of a valve system arranged between a camshaft and a valve for opening/closing an intake port or an exhaust port in an internal combustion engine, wherein the rocker arm comprises: a support hole for enabling rockable supporting of the rocker arm by a rocker shaft supported on a cylinder head; and a cam follower receiving a pushing force from a cam of the camshaft, the support hole and the cam follower being supported by a rolling bearing structure; a rolling bearing for supporting the support hole includes a bearing having a plurality of needles mounted on an inner side of an outer peripheral shell, the outer peripheral shell being fixed to the support hole; the cam follower comprises an outer ring supported on a periphery of a cam follower shaft held by a pair of left and right arm portions of the rocker arm by a plurality of rollers, a position of the outer ring and a position of the rollers in the lateral direction being restricted by the arm portion, and an outer diameter of the outer ring is set equal to or less than an inner diameter of the support hole.
 2. The structure of the rocker arm according to claim 1, wherein an escape portion is formed on the rocker arm so that the rocker arm does not obstruct the movement of a distal end of a cam nose when the camshaft is rotated.
 3. The structure of the rocker arm according to claim 1, wherein in an exhaust-side rocker arm out of the rocker arm, a decompression device is engaged with the exhaust-side rocker arm and is mounted on the camshaft, a decompression shaft portion of a decompression cam which the decompression device includes is supported on an inner side of an outer peripheral surface of a base circle portion of the exhaust cam, and the decompression cam applies a decompression action to a projecting portion of an arm portion of the exhaust-side rocker arm.
 4. The structure of the rocker arm according to claim 2, wherein in an exhaust-side rocker arm out of the rocker arm, a decompression device is engaged with the exhaust-side rocker arm and is mounted on the camshaft, a decompression shaft portion of a decompression cam which the decompression device includes is supported on an inner side of an outer peripheral surface of a base circle portion of the exhaust cam, and the decompression cam applies a decompression action to a projecting portion of an arm portion of the exhaust-side rocker arm.
 5. The structure of the rocker arm according to claim 3, wherein the arm portions of the exhaust-side rocker arm are formed wherein, compared to a width of the arm portion on a side where the arm portions is brought into contact with the decompression cam, a width of the arm portion on the other side is set small.
 6. The structure of the rocker arm according to claim 2, wherein the internal combustion engine is an internal combustion engine where the camshaft and the rocker shaft are arranged in front of a combustion chamber and in a planar projection view as viewed in the cylinder axis direction, with respect to each rocker arm, a size between an axis of the rocker shaft and an axis of the cam follower shaft is set smaller than a size between the axis of the rocker shaft and an axis of the camshaft.
 7. The structure of the rocker arm according to claim 5, wherein the rocker shaft is fixed to the cylinder head by fastening in a non-rotatable manner.
 8. The structure of the rocker arm according to claim 1, wherein the cam follower shaft of the cam follower is formed of a solid small-diameter shaft and a thickness of the outer ring in the radial direction is set smaller than a diameter of the roller.
 9. The structure of the rocker arm according to claim 2, wherein the cam follower shaft of the cam follower is formed of a solid small-diameter shaft and a thickness of the outer ring in the radial direction is set smaller than a diameter of the roller.
 10. The structure of the rocker arm according to claim 4, wherein the respective rocker arms arranged adjacent to each other are formed such that a width of the arm portion on a left side and a width of the arm portion on a right side differ from each other and the width of the inner arm portion is set smaller than a width of the outer arm portion.
 11. The structure of the rocker arm according to claim 1, wherein the internal combustion engine is a frontwardly-inclined engine wherein a cylinder axis extends frontwardly and upwardly and oil supply guide walls which lead to an end surface of the support hole of the rocker arm from the rocker shaft support boss portion in a vehicle mounted state are formed in the vicinity of the rocker shaft support boss portions of the cylinder head.
 12. The structure of the rocker arm according to claim 9, wherein the oil supply guide walls are arranged close to the rocker shaft support boss portions positioned laterally outside the intake-side rocker arm and the exhaust-side rocker arm arranged parallel to each other in the lateral direction toward an oil supply hole formed in the cylinder head cover.
 13. A rocker arm for a valve system arranged between a camshaft and a valve for opening/closing an intake port or an exhaust port in an internal combustion engine, comprising: a support hole formed for rockable supporting the rocker arm on a rocker shaft supported on a cylinder head; and a cam follower receiving a pushing force from a cam of the camshaft, the support hole and the cam follower being supported by a rolling bearing structure; a rolling bearing mounted within the support hole includes a plurality of needles mounted on an inner side of an outer peripheral shell, the outer peripheral shell being fixed to the support hole; the cam follower includes an outer ring supported on a periphery of a cam follower shaft held by a pair of left and right arm portions of the rocker arm by a plurality of rollers, a position of the outer ring and a position of the rollers in the lateral direction being restricted by the arm portion, and an outer diameter of the outer ring is set equal to or less than an inner diameter of the support hole.
 14. The rocker arm according to claim 13, wherein an escape portion is formed on the rocker arm so that the rocker arm does not obstruct the movement of a distal end of a cam nose when the camshaft is rotated.
 15. The rocker arm according to claim 13, wherein in an exhaust-side rocker arm out of the rocker arm, a decompression device is engaged with the exhaust-side rocker arm and is mounted on the camshaft, a decompression shaft portion of a decompression cam which the decompression device includes is supported on an inner side of an outer peripheral surface of a base circle portion of the exhaust cam, and the decompression cam applies a decompression action to a projecting portion of an arm portion of the exhaust-side rocker arm.
 16. The rocker arm according to claim 14, wherein in an exhaust-side rocker arm out of the rocker arm, a decompression device is engaged with the exhaust-side rocker arm and is mounted on the camshaft, a decompression shaft portion of a decompression cam which the decompression device includes is supported on an inner side of an outer peripheral surface of a base circle portion of the exhaust cam, and the decompression cam applies a decompression action to a projecting portion of an arm portion of the exhaust-side rocker arm.
 17. The rocker arm according to claim 15, wherein the arm portions of the exhaust-side rocker arm are formed wherein, compared to a width of the arm portion on a side where the arm portions is brought into contact with the decompression cam, a width of the arm portion on the other side is set small.
 18. The rocker arm according to claim 14, wherein the internal combustion engine is an internal combustion engine where the camshaft and the rocker shaft are arranged in front of a combustion chamber and in a planar projection view as viewed in the cylinder axis direction, with respect to each rocker arm, a size between an axis of the rocker shaft and an axis of the cam follower shaft is set smaller than a size between the axis of the rocker shaft and an axis of the camshaft.
 19. The structure of the rocker arm according to claim 17, wherein the rocker shaft is fixed to the cylinder head by fastening in a non-rotatable manner.
 20. The structure of the rocker arm according to claim 13, wherein the cam follower shaft of the cam follower is formed of a solid small-diameter shaft and a thickness of the outer ring in the radial direction is set smaller than a diameter of the roller. 